1. Field of the Invention
The present invention relates to an immersion nozzle for introducing molten steel from a tundish into a continuous casting mold, and more particularly to a structure of an immersion nozzle.
2. Description of the Prior Art
Deposits of oxide inclusion onto an inwall of an immersion nozzle increase in proportion to time lapse so much that the deposits not only restrict casting time but also builds a few microns of dioxide products contained in molten steel, resulting in often inducing defects of produced steel. This deposit of those inclusions is greatly affected by materials used in the immersion nozzle. For example, when an immersion nozzle is made of fused silica, there is almost no deposit of inclusions on the inwall of the immersion nozzle to be found. This immersion nozzle of fused silica, however, reacts with Mn or the like contained in the molten steel, and it is partially melted and damaged. Because of the melting loss, operating troubles may occur and the quality of cast steel products is unfavorably affected. In the ordinary case of casting aluminum killed steel, an immersion nozzle made of alumina graphite or of alumina graphite-zirconium is used. When an alumina graphite immersion nozzle is used, the deposit of oxide inclusions onto the nozzle inwall, sintering of the inclusions and growth thereof proceeds rapidly. Therefore, argon gas as an inert gas is blown in into the immersion nozzle to reduce the deposit of inclusion. Recently most immersion nozzles in use are made of zirconium because use thereof reduces the deposit of dioxide products.
FIGS. 1(a)-1(c) of the drawing show sectional views of an immersion nozzle of a .first prior art apparatus. FIG. 1(a) is a sectional plan view of the immersion nozzle taken on line 2--2 of FIG. 1(b), passing through the respective centers of exit ports 12a and 12b. FIG. 1(b) is a vertical section of the immersion nozzle taken on line 3--3 of FIG. 1(a). FIG. 1(c) is a vertical section of the immersion nozzle taken on line 4--4 of FIG. 1(c). Immersion nozzle body 10 of the first prior art immersion nozzle has bore 14 for passing molten steel therein and is provided with two exit ports 12a and 12b located symmetrically about the vertical center axis of the immersion nozzle body at a lower portion thereof. The cross-sectional area of bore 14 is the same over the whole length of the immersion nozzle body. The horizontal inner length of exit portions 12a and 12b is the same as that of bore 14. The immersion nozzle body is made of alumina graphite or zirconium. Reference numeral 16 denotes inclusions, particularly alumina deposited on the inwall of the immersion nozzle body as schematically illustrated in the drawing. Because the alumina deposit often flakes or drops off into the molten steel, defects of cast steel products sometime occur. In addition, the alumina deposit reduces the section areas of the bore of the immersion nozzle and the exit ports of the immersion nozzle, and increases flow speed of the molten steel from the exit ports into the mold. As a result, the molten steel makes an active movement and the surface level up-and-down movement of the molten steel is increased. The molten steel flows into the strand, accompanying mold powders floating on the surface of the molten steel in the mold and due to this, this prior art apparatus is disadvantageous in causing defects of cast steel products attributable to the mold powder.
FIGS. 2(a)-2(c) shows sectional views of an immersion nozzle of a second prior art apparatus. FIG. 2(a) is a sectional plan view of the immersion nozzle taken on line 2--2 of FIG. 2(b), passing through the respective centers of exit ports 12a and 12b. FIG. 2(b) is a vertical section of the immersion nozzle taken on line 3--3 of FIG. 2(a). FIG. 2(c) is a vertical section of the immersion nozzle taken on line 4--4 of FIG. 2(a). In this second prior art apparatus, argon gas is blown in into the molten steel through slit nozzle 20 set into the bottom portion 18 of the immersion nozzle body 10. In order to reduce the thickness of alumina deposited on the inwall thereof, however, this prior art immersion nozzle is required to blow in a large amount of argon gas not only through slit nozzle 20 set in the bottom portion but also through the top of the immersion nozzle body. The total amount of argon gas blown in the immersion nozzle is 12-20 Nl/min wherein N refers to a condition of room temperature and atmospheric pressure. Due to increase of the argon gas blow-in amount the cast steel products are easy to have surface defects of slag inclusions and blow holes. The slag inclusions arise from the surface level movement of the molten steel in the mold caused by bubbles and the blow holes are caused by not only the increase of the actual amount of argon gas but also the growth of the bubbles.